ESM 301_BSES_The Ecosystem Concept.pdf

The Ecosystem Concept ESM 301 | Terrestrial Ecosystem & Management Ecosystem Ecology studies the links between organisms and their physical environment within an Earth-System context addresses the interactions between organisms and their environment as an integrated system Ecosyste...

The Ecosystem Concept ESM 301 | Terrestrial Ecosystem & Management Ecosystem Ecology studies the links between organisms and their physical environment within an Earth-System context addresses the interactions between organisms and their environment as an integrated system Ecosystem Ecology The ecosystem approach is fundamental to managing Earth’s resources because it addresses the interactions that link biotic systems, of which people are an integral part, with the physical systems on which they depend. The approach applies at the scale of Earth as a whole, the Amazon River basin, or a farmer’s field. An ecosystem approach is critical to the sustainable management and use of resources in an era of increasing human population and consumption and large, rapid changes in the global environment. Ecosystem Ecology The ecosystem approach has grown in importance in many areas. The United Nations Convention on Biodiversity of 1992, for example, promoted an ecosystem approach, including humans, for conserving biodiversity rather than the more species-based approaches that predominated previously. A Focal Issue Human exploitation of Earth’s ecosystems has increased more in the last half-century than in the entire previous history of the planet (Steffen et al. 2004), often with unintended detrimental effects. Forest harvest, for example, provides essential wood and paper products (Fig. 1.1). The amount and location of harvest, however, influences other benefits that society receives from forests, including the quantity and quality of water in headwater streams; the recreational and aesthetic benefits of forests; the probability of landslides, insect outbreaks, and forest fires; and the potential of forests to release or sequester carbon dioxide (CO2), which influences climatic change. A Focal Issue Concern about loss of old growth forest habitat for endangered species such as the spotted owl led to the development of ecosystem management in the 1990s to address the multiple functions and uses of forests (Christensen et al. 1996; Szaro et al. 1999). Ecosystem ecology draws on a breadth of disciplines to provide the principles needed to understand the consequences of society’s choices.  01 Overview of ecosystem ecology Ecosystem processes and dynamics Overview of Ecosystem Ecology 1. The flow of energy and materials through organisms and the physical environment provides a framework for understanding the diversity of form and functioning of Earth’s physical and biological processes. Overview of Ecosystem Ecology Why do tropical forests have large trees but accumulate only a thin layer of dead leaves on the soil surface, whereas tundra supports small plants but an abundance of organic matter at the soil surface? Why does the concentration of carbon dioxide in the atmosphere decrease in summer and increase in winter? What happens to nitrogen fertilizer that farmers add to their fields but do not harvest with the crop? Why has the introduction of exotic grasses to pastures caused adjacent forests to burn? These are representative of the questions addressed by ecosystem ecology. Answers to these questions require an understanding of the interactions between organisms and their physical environments – both the response of organisms to environment and the effects of organisms on their environment. These questions also require a focus on integrated ecological systems rather than individual organisms or physical components. Overview of Ecosystem Ecology 2. Ecosystem analysis seeks to understand the factors that regulate the pools (quantities) and fluxes (flows) of materials and energy through ecological systems. These materials include carbon, water, nitrogen, rock- derived elements such as phosphorus, and novel chemicals such as pesticides or radionuclides that people have added to the environment. These materials are found in abiotic (nonbiological) pools such as soils, rocks, water, and the atmosphere and in biotic pools such as plants, animals, and soil microorganisms (microbes). Overview of Ecosystem Ecology An ecosystem consists of all the organisms and the abiotic pools with which they interact. Ecosystem processes are the transfers of energy and materials from one pool to another. Overview of Ecosystem Ecology Energy enters an ecosystem when light energy drives the reduction of carbon dioxide (CO2) to form sugars during photosynthesis. Organic matter and energy are tightly linked as they move through ecosystems. The energy is lost from the ecosystem when organic matter is oxidized back to CO2 by combustion or by the respiration of plants, animals, and microbes. Overview of Ecosystem Ecology Materials move among abiotic components of the system through a variety of processes, including the weathering of rocks, the evaporation of water, and the dissolution of materials in water. Fluxes involving biotic components include the absorption of minerals by plants, the fall of autumn leaves, the decomposition of dead organic matter by soil microbes, the consumption of plants by herbivores, and the consumption of herbivores by predators. Overview of Ecosystem Ecology Most of these fluxes are sensitive to environmental factors such as temperature and moisture, and to biological factors regulating the population dynamics and species interactions in communities. The unique contribution of ecosystem ecology is its focus on biotic and abiotic factors as interacting components of a single integrated system. Overview of Ecosystem Ecology 3. Ecosystem processes can be studied at many spatial scales. (How big is an ecosystem?) Ecosystem processes take place at a wide range of scales, but the appropriate scale of study depends on the question asked (Fig. 1.2). ▪ The impact of zooplankton on their algal food might be studied in small bottles in the laboratory. ▪ The controls over productivity might be studied in relatively homogeneous patches of a lake, forest, or agricultural field. ▪ Questions that involve exchanges occurring over very broad areas might best be addressed at the global scale. Overview of Ecosystem Ecology 4. Ecosystem dynamics are a product of many temporal scales. The rates of ecosystem processes are constantly changing due to fluctuations in environment and activities of organisms on time scales ranging from microseconds to millions of years. Many early studies in ecosystem ecology made the simplifying assumption that some ecosystems are in equilibrium with their environment. In this perspective, relatively undisturbed ecosystems were thought to have properties that reflected (1) largely closed systems dominated by internal recycling of elements, (2) self- regulation and deterministic dynamics, (3) stable endpoints or cycles, and (4) absence of disturbance and human influence (Pickett et al. 1994; Turner et al. 2001). Overview of Ecosystem Ecology Ecosystems are considered to be at steady state, if the balance between inputs and outputs to the system shows no trend with time (Bormann and Likens 1979). Steady state assumptions differ from equilibrium assumptions because they accept temporal and spatial variation as a normal aspect of ecosystem dynamics. Overview of Ecosystem Ecology 5. Ecosystem ecology depends on information and principles developed in physiological, evolutionary, population, and community ecology (Fig. 1.3). The biologically mediated movement of carbon and nitrogen through ecosystems depends on the physiological properties of plants, animals, and soil microbes. The traits of these organisms are the products of their evolutionary histories and the competitive interactions that sort species into communities where they successfully grow, survive, and reproduce (Vrba and Gould 1986). Ecosystem fluxes also depend on the population processes that govern plant, animal, and microbial densities and age structures and on community processes such as competition and predation that determine which species are present and their rates of resource consumption. Overview of Ecosystem Ecology People interact with ecosystems through both their impacts on ecosystems and their use of ecosystem services – the benefits that people derive from ecosystems. The patterns of human engagement with ecosystems reflect a complex suite of social processes operating at many temporal and spatial scales. Overview of Ecosystem Ecology Ecosystem ecology therefore informs and depends on concepts in the emerging field of social–ecological stewardship that enables people to shape the trajectory of social– ecological change to enhance ecosystem resilience and human well-being (Fig. 1.3).  02 History of ecosystem ecology Early discoveries of ecosystem processes History of Ecosystem Ecology Many early discoveries of biology were motivated by questions about the integrated nature of ecological systems. In the seventeenth century, European scientists were still uncertain about the source of materials found in plants. ▪ Plattes, Hooke, and others advanced the novel idea that plants derive nourishment from both air and water (Gorham 1991). ▪ Priestley extended this idea in the eighteenth century by showing that plants produce a substance that is essential to support the breathing of animals. ▪ At about the same time, MacBride and Priestley showed that breakdown of organic matter caused production of “fixed air” (carbon dioxide) that did not support animal life. ▪ In the nineteenth century, De Saussure, Liebig, and others clarified the explicit roles of carbon dioxide, oxygen, and mineral nutrients in these cycles. History of Ecosystem Ecology Much of the biological research during the nineteenth and twentieth centuries explored the detailed mechanisms of biochemistry, physiology, behavior, and evolution that explain how life functions. Many threads of ecological thought have contributed to the development of ecosystem ecology (Hagen 1992), including ideas relating to trophic interactions (the feeding relationships among organisms) and biogeochemistry (biological interactions with chemical processes in ecosystems). Early research on trophic interactions emphasized the transfer of energy among organisms. History of Ecosystem Ecology Elton, an English zoologist interested in natural history, described the role that an animal plays in a community (its niche) in terms of what it eats and is eaten by (Elton 1927). He viewed each animal species as a link in a food chain that describes the movement of matter from one organism to another. Elton’s concepts of trophic structure provide a framework for understanding the flow of materials through ecosystems. History of Ecosystem Ecology Meanwhile, Tansley, a British terrestrial plant ecologist, was also concerned that ecologists focused their studies so strongly on organisms that they failed to recognize the importance of exchange of materials between organisms and their abiotic environment. He coined the term ecosystem to emphasize the importance of interchanges of materials between organisms and their environment (Tansley 1935). History of Ecosystem Ecology Lindeman, another limnologist, was strongly influenced by all these threads of ecological theory. He suggested that energy flow through an ecosystem could be used as a currency to quantify the roles that groups of organisms play in trophic dynamics. Green plants (primary producers) capture energy and transfer it to animals (consumers) and decomposers. At each transfer, some energy is lost from the ecosystem through respiration. Therefore, the productivity of plants constrains the quantity of consumers that an ecosystem can support. History of Ecosystem Ecology H.T. Odum, also trained by Hutchinson, and his brother E.P. Odum further developed the “systems approach” to studying ecosystems, emphasizing the general properties of ecosystems without documenting all the underlying mechanisms and interactions. Some of the questions addressed by systems ecology include information transfer (Margalef 1968), the structure of food webs (Polis 1991), the hierarchical changes in ecosystem controls at different temporal and spatial scales (O’Neill et al. 1986; Peterson et al. 1998; Enquist et al. 2007), and the resilience of ecosystem properties after disturbance (Holling 1973). History of Ecosystem Ecology Additionally, regional and global changes in the environment have increased ecologists’ awareness of the effects of disturbance and other environmental changes on ecosystem processes. Succession, the directional change in ecosystem structure and functioning that follows disturbance, is an important framework for understanding these transient dynamics of ecosystems. Clements advanced a theory of community development, suggesting that this vegetation succession is a predictable process that eventually leads, in the absence of disturbance, to a stable community-type characteristic of a particular climate (the climatic climax; Clements 1916). He suggested that a community is like an organism made of interacting parts (species) and that successional development toward a climax community is analogous to the development of an organism to adulthood. History of Ecosystem Ecology These information provides hints about which ecosystems and processes have the greatest impact on the Earth System and therefore where research and management should focus efforts to understand and solve these problems. In short, the intersection of systems approaches, process understanding, and global analysis is an exciting frontier of ecosystem ecology.  03 Ecosystem structure and functioning Ecosystem processes and structure Ecosystem Processes Most ecosystems gain energy from the sun and materials from the air or rocks, transfer these among components within the ecosystem, then release energy and materials to the environment. The essential biological components of ecosystems are plants, animals, and decomposers. The essential abiotic components of a terrestrial ecosystem are water, the atmosphere, which supplies carbon and nitrogen, and soil, which provides support, storage, and other nutrients required by organisms. Ecosystem Processes Decomposer microorganisms (microbes) break down dead organic material, releasing CO2 to the atmosphere and nutrients in forms that are available to other microbes and plants. Animals transfer energy and materials and can regulate the quantity and activities of plants and soil microbes. An ecosystem model describes the major pools and fluxes in an ecosystem and the factors that regulate these fluxes. Carbon, water, and nutrients differ from one another in the relative importance of ecosystem inputs and outputs vs. internal recycling. Ecosystem Processes The pool sizes and rates of cycling of carbon, water, and nutrients differ substantially among ecosystems. Tropical forests have much larger pools of carbon and nutrients in plants than do deserts or tundra. Peat bogs, in contrast, have large pools of soil carbon rather than plant carbon. Ecosystems also differ substantially in annual fluxes of materials among pools, for reasons that we explore in later chapters. Controls Over Ecosystem Processes Ecosystem structure and functioning are governed by multiple independent control variables. These state factors, as Jenny and his coworkers called them, include climate, parent material (the rocks that give rise to soils), topography, potential biota (the organisms present in the region that could potentially occupy a site), and time (Fig. 1.5; Jenny 1941; Amundson and Jenny 1997; Vitousek 2004). Together these five factors, among others, set the bounds for the characteristics of an ecosystem. Controls Over Ecosystem Processes ▪ On broad geographic scales, climate is the state factor that most strongly determines ecosystem processes and structure. Global variations in climate explain the distribution of biomes (general categories of ecosystems) such as wet tropical forests, temperate grasslands, and arctic tundra. ▪ Within each biome, parent material strongly influences the types of soils that develop and explains much of the regional variation in ecosystem processes. ▪ Topographic relief influences both microclimate and soil development at a local scale. ▪ The potential biota governs the types and diversity of organisms that actually occupy a site. Island ecosystems, for example, are often less diverse than climatically similar mainland ecosystems because new species reach islands less often and are more likely to go locally extinct than on the mainland (MacArthur and Wilson 1967). ▪ Time influences the development of soil and the evolution of organisms over long time scales (Vitousek 2004). Time also incorporates the influences on ecosystem processes of past disturbances and environmental changes over a wide range of time scales. Controls Over Ecosystem Processes ▪ A chronosequence, for example, is a series of sites of different ages with similar climate, parent material, topography, and potential to be colonized by the same organisms. ▪ In a toposequence, ecosystems differ mainly in their topographic position (Shaver et al. 1991). Sites that differ primarily with respect to climate or parent material allow us to study the impacts of these state factors on ecosystem processes (Vitousek 2004). Finally, a comparison of ecosystems that differ primarily in potential biota, such as the Mediterranean shrublands that have developed on west coasts of California, Chile, Portugal, South Africa, and Australia, illustrates the importance of evolutionary history in shaping ecosystem processes (Mooney and Dunn 1970; Cody and Mooney 1978). Controls Over Ecosystem Processes Ecosystem processes both respond to and control the factors that directly govern their activity. Interactive controls are factors that operate at the ecosystem scale and both control and respond to ecosystem characteristics (Fig. 1.5; Chapin et al. 1996). Important interactive controls include the supply of resources to support the growth and maintenance of organisms, microenvironment (e.g., temperature, pH) that influences the rates of ecosystem processes, disturbance regime, and the biotic community. Controls Over Ecosystem Processes Landscape-scale disturbance by fire, wind, floods, insect outbreaks, and hurricanes is a critical determinant of the natural structure and process rates in ecosystems (Pickett and White 1985; Peters et al. 2011). Like other interactive controls, disturbance regime depends on both state factors and ecosystem processes. Fire probability and spread, for example, depends on both climate and the quantity and flammability of plants and dead organic matter. Deposition and erosion during floods shape river channels and influence the probability of future floods. Change in either the intensity or frequency of disturbance can cause long-term ecosystem change. Woody plants, for example, often invade grasslands when fire suppression reduces fire frequency. Controls Over Ecosystem Processes The nature of the biotic community – i.e., the types of species present, their relative abundances, and the nature of their interactions, can influence ecosystem processes just as strongly as do differences in climate or parent material. These species effects can often be generalized at the level of functional types, which are groups of species that are similar to one another in their role in a specific community or ecosystem process. Feedbacks regulate the internal dynamics of ecosystems. A thermostat, for example, causes a furnace to switch on when a house gets cold and to switch off when the house warms to the desired temperature. Natural ecosystems are complex networks of interacting feedbacks (DeAngelis and Post 1991). Controls Over Ecosystem Processes Stabilizing feedbacks (termed negative feedbacks in the systems literature) occur when two components of a system have opposite effects on one another (Fig. 1.6). Consumption of prey by a predator, for example, has a positive effect on the consumer but a negative effect on the prey. The negative effect of predators on prey prevents uncontrolled growth of a prey’s population, thereby stabilizing the population sizes of both predator and prey. There are also amplifying feedbacks (termed positive feedbacks in the systems literature) in ecosystems in which both components of a system have a positive effect on one other, or both have a negative effect on one another. Plants, for example, provide their mycorrhizal fungi with carbohydrates in return for nutrients. This exchange of growth-limiting resources between plants and fungi promotes the growth of both components of the symbiosis until they become constrained by other factors.  04 Human-induced ecosystem change Human impacts on ecosystems, degradation on ecosystem changes, resilience & thresholds Human Impacts on Ecosystems Human activities have transformed the land surface, species composition, and biogeochemical cycles at scales that have altered the biogeochemistry and climate of the planet. These anthropogenic (human-caused) effects are so profound that the beginning of the industrial revolution (about 1,750) is widely recognized as the start of a new geologic epoch – the Anthropocene (Crutzen 2002). The most direct and substantial human alteration of ecosystems is through the transformation of land for production of food, fiber, and other goods used by people (Fig. 1.7). Human activities have also altered freshwater and marine ecosystems. Human Impacts on Ecosystems Land-use change and the resulting loss of habitat are the primary driving forces causing species extinctions and loss of biological diversity (Mace et al. 2005). In addition, transport of species around the world increases the frequency of biological invasions, due to the globalization of the economy and increased international transport of people and products. Others alter the structure and functioning of ecosystems, leading to further loss of species diversity. Many biological invasions are irreversible because it is difficult or prohibitively expensive to remove invasive species once they establish. Human Impacts on Ecosystems Human activities have influenced biogeochemical cycles in many ways. Extensive use of fossil fuels and the expansion and intensification of agriculture have increased the concentrations of atmospheric gases, altering global cycles of carbon, nitrogen, phosphorus, sulfur, and water. Human Impacts on Ecosystems Human activities introduce novel chemicals into the environment. Some apparently harmless anthropogenic gases have had drastic impacts on the atmosphere and ecosystems. Chlorofluorocarbons (CFCs), for example, were first produced in the 1950s as refrigerants, propellants, and solvents. In the upper atmosphere, however, CFCs react with and deplete ozone, which shields Earth’s surface from high-energy UV radiation. Ozone depletion was first detected as a dramatic ozone hole near the South Pole, but it now occurs at lower latitudes in the southern hemisphere and at high Northern latitudes. Human Impacts on Ecosystems Other synthetic organic chemicals include DDT (an insecticide) and PCBs (polychlorinated biphenyls, industrial compounds) that were used extensively in the developed world in the 1960s before their ecological impacts were widely recognized. They are mobile and degrade slowly, causing long-term persistence and transport to ecosystems across the globe. Many of these compounds are fat soluble, so they accumulate in organisms and increase in concentration as they move up food chains. Human Impacts on Ecosystems When these compounds reach critical concentrations, they can cause reproductive failure (Carson 1962), particularly in higher trophic levels and in animals that feed on fat-rich species. Some processes, such as eggshell formation in birds, are particularly sensitive to pesticide accumulations and have caused population declines in predatory birds like the peregrine falcon, even in regions far removed from the locations of pesticide use. Human Impacts on Ecosystems Atmospheric testing of atomic weapons in the 1950s and 1960s increased atmospheric concentrations of radioactive forms of many elements. Explosions and leaks in nuclear reactors used to generate electricity have also released radioactivity at local to regional scales. The explosion of a power-generating plant in 1986 at Chernobyl in the Ukraine, for example, released radioactivity that directly affected human health in the region and increased the atmospheric deposition of radioactive materials across Eastern Europe and Scandinavia. Human Impacts on Ecosystems In other cases, the chemicals that people introduce to ecosystems are much more targeted as in the case of BT-corn, a genetically modified corn variety carrying bacterial genes that cause production of a compound that is toxic to European corn borer. Any introduction of novel chemicals raises questions of toxicity to non-target organisms or the evolution of resistance in target species (Marvier et al. 2007). These questions are amenable to study by ecosystem ecologists. Human Impacts on Ecosystems The growing scale and extent of human activities suggest that all ecosystems are being influenced, directly or indirectly, by human actions. No ecosystem functions in isolation, and all are influenced by human activities taking place in adjacent communities and around the world. Human activities are leading to global changes in most major ecosystem controls: climate (global warming), soil and water resources (nitrogen deposition, erosion, diversions), disturbance regime (land-use change, fire suppression), and functional types of organisms (species introductions and extinctions). These changes in interactive controls inevitably alter ecosystem dynamics. Resilience and Threshold Changes Despite pervasive human impacts on state factors and interactive controls, ecosystems exhibit a wide range of responses, ranging from substantial resilience to threshold changes. Resilience is the capacity of a social–ecological system to maintain similar structure, functioning, and feedbacks despite shocks and perturbations. Thresholds are critical levels of one or more ecosystem controls that, when crossed, cause abrupt ecosystem changes. Biodiversity can confer resilience because a large number of species is likely to sustain ecosystem processes over a broader range of conditions than would one or a few species. Resilience and Threshold Changes Social processes that govern the role of people in ecosystems can be a source of resilience (sustainability) or can trigger threshold changes. Ecologists are only beginning to understand the factors that govern ecosystem resilience and threshold change. This is emerging as a critical research area in our increasingly human- dominated planet. Although some pressures on ecosystems are easily observed (e.g., acid rain) or predicted (e.g., rising global temperature that was predicted decades ago and is now being observed), surprises that are difficult or impossible to anticipate also occur. Thanks! 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